The pluralization is correct. I would argue about the veracity of the statement though.
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JimApr 7 '12 at 3:28

I agree with @Jim, and would add that I'm not sure why you'd ask such a question. Why would anyone say "... lists is ...", or am I missing your question?
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Amos M. CarpenterApr 7 '12 at 3:38

Is the use of "a thing" right? Why not "things"?
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qazwsxApr 7 '12 at 3:56

1

There is no grammatical requirement when you say A are/is B that A and B have the same number. The grammatical requirement is that the verb agree with A. So, for example, "The forged documents are a problem" is correct, and I believe would be much more likely to be said by a native speaker than "The forged documents are problems".
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Peter Shor Apr 7 '12 at 12:55

4 Answers
4

As I believe your intuition says, "are things of the past" is much more common than "are a thing of the past". Consider the Google Ngram below:

And looking at the actual citations, a reasonable fraction (at least a quarter of them) are grammatical constructions parallel to "to-do lists are things of the past".

However, Google still finds many, many examples parallel to "to-do lists are a thing of the past". These are grammatically correct because in the construction A is/are B, English makes no requirement that A and B have the same number; the requirement is that the verb agrees with A. For a different example, I think that native English speakers would be more likely to say "to-do lists are a problem" than "to-do lists are problems". You can use Ngrams to check that this is currently true (it's changed from 50 years ago) if you replace to-do lists with men, women, or people.

Why the more grammatically correct sentence is less used in practice?
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qazwsxApr 7 '12 at 4:13

Made an edit to account for the first example being technically grammatically incorrect, though it maintains the idiomatic expression verbatim. I would argue it's completely acceptable to modify the number in idiomatic expressions, though, especially since idioms (and English in general) do not have an authoritative guide other than usage among speakers, anyway.
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Charlie GorichanazApr 7 '12 at 4:14

1

Also, the Google search was not capable of capturing which cases of "a thing of the past" correspond to plural nouns in the subject. So it may very well be uncommon to have your exact case written that way.
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Charlie GorichanazApr 7 '12 at 4:15

This is why (American) English is so hard to learn. You impose such rules like pluralization of nouns etc. etc. but often just randomly make exceptions. And there is no way to verify if it's correct or not, and no way to know what exceptions are "acceptable." Is this (Usage 1) a thing that is basically a mistake made by many people and/or the majority?
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qazwsxApr 7 '12 at 4:20

You're correct. There is no authority on English other than speaker consensus. Generally you won't be wrong if you have a logical reason for what you do, such as making your nouns agree in number. Occasionally an illogical form will become popular in use, and over time becomes "acceptable" even if it's not logical. I try to avoid such situations, though!
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Charlie GorichanazApr 7 '12 at 4:23

Does a noun-like idiomatic expression typically not have both singular and plural forms? I.e. a singular idiomatic expression is not considered and/or used as an idiomatic expression once plurized? What about the other way around?
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qazwsxApr 7 '12 at 4:17

1

No: an idiomatic expression is an idiom. Change it and it ceases to be idiomatic. A thing of the past refers to the entire class of "To-do lists"; things of the past needs two separate "things". One might say "to-do lists and notebooks are things of the past", perhaps.
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Andrew Leach♦Apr 7 '12 at 12:44

@Andrew: Sometimes, for some idiomatic expressions. But Google shows lots of hits of the form "sthgs are things of the past" where "things of the past" is the plural of the idiomatic expression "a thing of the past".
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Peter Shor Apr 7 '12 at 14:47

The typical constructions are: "the afternoon newspaper is a thing of the past" or "afternoon newspapers are things of the past". You do also occasionally see "afternoon newspapers are a thing of the past". You need to put in the "are" to make sure the subject is a plural noun.
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Peter Shor Apr 7 '12 at 16:16